Scientists said the discovery of changes in the blood which signify the earliest stages of disease could herald a major breakthrough - potentially leading to successful treatment during a critical “window of opportunity” .

The study found biomarkers in the blood could be used to forecast whether a person would develop Alzheimer’s disease within three years, with a 90 per cent level of accuracy.

At an international summit on dementia last month, Jeremy Hunt, the Health Secretary, said the development of such tests could constitute “a massive step forward” in the battle against the disease.

Charities on Sunday said the findings from the new US trial were encouraging and could signify an important advance in dementia treatment.

However, they also warned of ethical dilemmas ahead - and said patients must be given a choice about whether they wanted to receive potentially devastating news about their future.

The research, published in the journal Nature Medicine, identified 10 molecules in the blood which could be used to predict with at least 90 per cent accuracy whether people went on to develop mild cognitive impairment or Alzheimer’s disease.

It is the first study to show differences in biomarkers in the blood between those who went on to suffer the disease, and those who remained “cognitively normal”.

Researchers said blood tests to identify those likely to develop dementia could be used in major clinical trials within two years.

Scientists from Georgetown University Medical Centre in the US examined 525 healthy participants aged 70 and over and monitored them for five years.

They regularly analysed blood samples from 53 participants who developed dementia, comparing them with those from 53 “cognitively normal” people.

When they tracked back, they found that the biomarkers could predict with 90 per cent accuracy whether participants went on to develop Alzheimer’s disease or mild cognitive impairment.

Professor Howard Federoff, one of the study’s authors, and professor of neurology at Georgetown University Medical Centre, said: “The lipid panel was able to distinguish with 90 per cent accuracy these two distinct groups: cognitively normal participants who would progress to mild cognitive impairment or Alzheimer’s disease within two to three years, and those who would remain normal in the near future.”

The researchers are now designing a further study in which those forecast with a high risk of Alzheimer’s disease are given access to experimental treatments which might delay or prevent the condition.

One in three people will develop dementia in their lifetime, and a rapidly ageing population means numbers are forecast to double within two decades.

More than 800,000 people in the UK have a form of dementia - most commonly Alzheimer’s disease - but Government estimates suggest that just half of cases receive a diagnosis.

Existing drugs for Alzheimer’s disease work best if given early, and can lessen symptoms, but do not slow overall progression of disease.

Scientists believe the best hope of finding a breakthrough treatment, which can slow or reverse progression of the condition, may come if drugs can be given long before patients show signs of disease.

Prof Federoff said that being able to predict disease before it was symptomatic could offer a “window of opportunity” which could be critical to developing effective drugs to combat disease before it took hold.

Researchers said they believed the results were “a major step forward” towards introducing large-scale screening to identify at-risk individuals, and test possible treatments and preventative drugs.

Dr Simon Ridley, head of research at charity Alzheimer’s Research UK, said: “Alzheimer’s disease begins to develop long before symptoms such as memory loss appear, but detecting the disease at this pre-symptomatic stage has so far proved difficult. More work is needed to confirm these findings, but a blood test to identify people at risk of Alzheimer’s could be an important finding for research.”

He said further studies were needed on larger groups of people, to see if treatments were more effective if given earlier.

“These are encouraging findings, but it’s vital to continue investing in research to capitalise on results like these,” he said.

Dr Doug Brown, director of research and development at the Alzheimer’s Society, said: “This is a really welcome development - it takes us a step closer to something that could impact on the lives of thousands of people with dementia, but we do need more research on larger groups and different populations.”

He said “significant ethical considerations” would have to be taken into account in offering any such test, particularly in doing so when an effective treatment has yet to be found.

“People must be given a choice about whether they would want to know, and whether they fully understand the implications of having such a test,” he said.